Since liquid crystal displays are lighter and more compact display devices than CRTs, they find extensive application in computers, electronic calculators, clocks, and watches. The principle of operation of liquid crystal displays depends on a change in an optical property, such as interference, scattering, diffraction, optical rotation, or absorption, of a liquid crystal material. This change is caused by a variation in orientation of the liquid crystal molecules or a phase transition in response to application of an external field such as electric field or heat.
Generally, a liquid crystal display comprises a pair of substrates with a given spacing which is maintained at 1 to tens of micrometers. A liquid crystal material is held between these substrates, thus forming a liquid crystal panel. At least one of the substrates has transparency. Electrodes are formed on both or one of the substrates. Using these electrodes, an electric field is applied to the liquid crystal material to control the orientation of the liquid crystal molecules in each different pixel within the substrate plane, thus controlling the amount of light transmitted through the liquid crystal panel. In this way, a desired image is displayed. The liquid crystal display is constructed differently according to which is the above-described optical properties is utilized, i.e., depending on the mode of operation. For example, polarizing plates are mounted on the outside of the liquid crystal panel.
To date, twisted-nematic (TN) liquid crystal displays and supertwisted-nematic (STN) liquid crystal displays enjoy wide acceptance. These kinds of liquid crystal displays make use of optical properties of liquid crystal materials such as optical rotation and interference of birefringent light. Both kinds require polarizing plates.
Where an image is displayed by the above-described liquid crystal display, numerous pixels must be controlled at the same time. For this purpose, various methods have been proposed. Among them, active matrix driving is used widely because it is a method capable of displaying image with high information content and high image quality. In particular, nonlinear active devices such as diodes or transistors are arranged at pixels. The individual pixels are electrically isolated from each other. Interference with unwanted signals is prevented. Thus, high image quality is accomplished. In this method, each pixel can be regarded as a capacitor to which an electrical switch is connected. Accordingly, the switches are turned on and off according to the need. As a result, electric charge can be made to go into and out of the pixels. If the switches are turned off, electric charge can be retained in the pixels and so the pixels can retain memory.
Problems with the Prior Art Techniques
(1) Electric Power Consumed by Liquid Crystal Display
In any kind of liquid crystal display, the liquid crystal material itself does not emit light. In order to have good visibility, a light source is incorporated in the equipment (transmissive type), or light incident on the equipment from surroundings is utilized (reflective type).
In the case of the transmissive type, if the intensity of light emitted by the light source is increased, then a brighter display device can be accomplished accordingly. However, this increases the power consumption of the whole apparatus. Most of the electric power is consumed by the light source in the transmissive type liquid crystal display. The ratio of the electric power consumed by the liquid crystal panel to the electric power consumed by the light source=1:100-1:1000. Hence, lower power consumption can be effectively accomplished by reducing the electric power expended by the light source. However, in both TN and STN types, it is customary to use two polarizing plates. This considerably lowers the transmittance of the liquid crystal panel. Therefore, in order to accomplish a bright display, it is necessary to increase the brightness of the light source. Since it is necessary to maintain a certain degree of brightness, a great reduction in the power consumption cannot be expected unless a light source of extremely high emission efficiency is employed.
On the other hand, in the case of the reflective type liquid crystal display, any special light source is not present inside the display device and so low power consumption and a reduction in size are enabled. Hence, it can be said that the reflective type is an ideal display device. However, it makes use of light coming from the surroundings. In order to achieve a brighter display device with a small amount of light, it is necessary to utilize the light efficiently. Since the reflective TN and STN types use no light sources, the brightness of the display devices is reduced accordingly.
(2) Response Speed
As image with higher information content is displayed, it is required that the response speed of the liquid crystal material be increased. However, the response time of the above-described TN liquid crystal display is tens of milliseconds. The response time of the STN liquid crystal display is on the order of 100 ms. At these response speeds, when the image created on the viewing screen moves across the screen, the image appears to tail off. In this way, the image quality is not good. In order to improve this phenomenon, it is necessary to utilize a liquid crystal material of higher response speed or to establish a mode of operation that accomplishes a higher response.
(3) Viewing Angle Characteristics
In both TN and STN types, when the liquid crystal display is viewed obliquely, a decrease in contrast, inversion of intermediate color hues, and other undesirable phenomena are observed, because the state of polarization of light entering obliquely changes in the liquid crystal layer of the liquid crystal display. In view of these problems, a method of dividing each pixel electrode into plural parts, a method of producing plural different states of orientation, and other methods have been proposed but no fundamental solution to the problems is available today.